This invention pertains generally to composite rotors, and more particularly to composite rotors for use in high torque applications.
Composite materials have been used for the past several decades in applications where structural elements encounter high levels of stress and/or temperature. In particular, these composite materials typically comprise fiber reinforcements and a matrix material (polymer, metal, or ceramic) that secures and condenses the fiber reinforcements. The fiber reinforcements are typically carbon fibers, but may be made of other materials, such as polymers, glass, metal, or ceramic. One such composite material is sometimes referred to as a fiber reinforced ceramic matrix composite, or FRCMC. These composites have become increasingly popular for applications where metals become structurally ineffective, such as applications involving principally high temperature environments, but also in combination with high speed rotations and high torque transmissions. In this regard, the very low density and high strength characteristics of FRCMCs at high temperatures make these composites particularly advantageous for the construction of high performance turbines. However, FRCMC components must be designed to withstand the high levels of torque associated with these types of turbines, which can require torque transmissions of over 5000 ft-lbs.
Structural elements, such as a turbine rotor, can be formed from FRCMC through machining or other processes. As stated above, FRCMC typically comprises a plurality of woven fiber plies in a two-dimensional plane. The plies are stacked on top of one another and may be attached by stitching, needling, or the like. The woven fiber plies are condensed by the ceramic matrix material using a gaseous or liquid process, such as chemical vapor infiltration.
Although the advantageous qualities of FRCMCs have been recognized for some time, early attempts at making turbine rotors using FRCMCs left much to be desired. In particular, the early designs were limited in the amount of torque that they could withstand. For example, FIG. 1 shows an early design of a FRCMC turbine rotor 100 that was successfully operational at torque levels of about 39 ft-lbs. The body portion 110 generally has a flat face surface, with radially extending blades 112 extending therefrom. A conical hub 114 protrudes away from the body portion 110, and three splines 116 are machined into the hub for engaging a coupler or gear (not shown). In operation, the turbine rotor 100 drives the coupler into rotation via the splines 116.
Although the turbine rotor 100 is structurally adequate for low torque applications, several disadvantages arise when considering high torque applications. In particular, the splines 116 are located at the inner diameter of the conical hub 114, which is where the highest centrifugal hoop stresses occur during operation for any rotating disk. In addition, the engagement of the coupler via the splines located at the inner diameter of the rotor provides the least radial leverage to carry the torque produced by the blades 112. In high torque applications, the turbine rotor of FIG. 1 would also likely have difficulties and could fail due to the high centrifugal hoop stresses and torque-induced inter-laminar shear stresses that arise during such applications. Furthermore, the hub 114 that spaces the splines 116 away from the body portion 110 creates a stress concentration point 120 at the intersection of the hub with the body portion 110. The stress concentration point 120 may lead to delamination and potential loss of torque transmission in high torque applications.
Another disadvantage of the turbine rotor of FIG. 1 is that the splines 116 are shaped such that the rotor 100 has only minimal or non-existent centering capability. More specifically, various cross-sections of the rotor 100 radially expand at different portions of the rates as the temperature of the rotor increases during operation. For example, a cross-section of the rotor 100 through the portion of the hub 114 containing the splines 116 expands radially at a different rate than a solid cross-section of the rotor through the body portion 110. As a result, the differential expansion may cause the rotor 100 to become unbalanced, which can damage or destroy the rotor.
A further disadvantage of the turbine rotor of FIG. 1 is that the splines 116 are formed in a manner that could cause delamination in high torque applications. In particular, each of the splines 116 includes sidewalls that are perpendicular to the flat face surface and, more importantly, perpendicular to the composite plies that form the rotor. Each spline also includes a base surface extending between the sidewalls and parallel to the flat face surface as well as the composite plies. Similar to the stress concentration point 120 created by the intersection of the hub 114 with the body portion 110, the splines have stress concentration points at the intersection of the sidewalls with the base surface. Thus, the stress concentration points created by the splines may lead to delamination and potential loss of torque transmission in high torque applications.
Other FRCMC rotors have been designed including an unbladed disk formed of carbon/silicon carbide FRCMC with a Gleason-machined curvic coupling that was developed by Rocketdyne. In addition, NASA is developing a carbon/silicon carbide FRCMC bladed disk with biconic friction couplers for a SIMPLEX turbopump. However, these other FRCMC rotors are also designed to withstand somewhat limited torque levels, such as 140 ft-lbs. or less. As will be apparent, these FRCMC turbine rotors are therefore designed to withstand torque levels that are at least an order of magnitude less than the 5,000+ft-lbs. of torque associated with some turbines.
The rotor of the present invention has a composite construction, yet is capable of torque transmission several orders of magnitude greater than previous composite turbine rotors. The rotor of the present invention is applicable for many types of applications, but is particularly advantageous in the field of rocket propulsion. Advantageously, the rotor of the present invention includes a plurality of recessed splines located in the body portion of the rotor. The recessed splines provide more shear load area between the rotor and a mated coupler for improved torque transmission. In one particularly advantageous embodiment, recessed splines may be included on both sides of the rotor to further increase the torque carrying capability of the rotor. As a result, the rotor of the present invention is capable of a torque transmission range of about 5,500-10,000 ft-lbs., which is orders of magnitude greater than previous composite turbine rotors.
In particular, the rotor of the present invention is disk-shaped and is formed of a composite material. Preferably, the composite material is a fiber-reinforced ceramic matrix composite, or FRCMC, that is known in the art and includes several plies of woven fiber reinforcement secured by a composite matrix. Other types of composite materials may also be used, such as polymeric or metallic. The rotor includes a main body portion that typically defines a central opening for receiving a shaft. The rotor also includes a plurality of circumferentially spaced turbine blades extending radially from the body portion. To provide strength and stiffness to the turbine blades, woven plies of fiber reinforcement continuously extend from the part of the body portion adjacent the central opening to the turbine blades.
As stated above, the rotor also includes a plurality of splines in the body portion thereof. The splines are adapted for engaging a coupler, such as a gearwheel, and transmitting torque thereto. In one embodiment, the rotor includes splines on both of the opposite sides of the rotor, although in another embodiment the splines are included only on one side of the rotor. However, by including the splines on both sides of the rotor, the torque carrying capability of the rotor is increased because the applied torque generated at the turbine blades is distributed to both sides of the rotor.
Advantageously, the splines are recessed into the generally planar surface of the body portion of the rotor instead of extending outwardly as in conventional designs. In this regard, the amount of shear area provided by the recessed splines for transmitting torque from the rotor to the coupler or other device is significantly increased because the shear area includes not only area in the inter-laminar direction, but also in the cross-laminar direction. In addition, a high number of radially spaced splines are provided about the central opening of the body portion to increase the overall shear area, thus reducing the shear stresses on each spline. Typically, the splines are radially elongate so as to have a length in the radial direction that is greater than the width of the spline in the circumferential direction. In one embodiment, the radially elongate splines have a radially tapering shape, such as a wedge shape, that further increases the amount of shear area per spline.
As stated above, the splines are recessed into the body portion and extend radially from the central axis. Furthermore, the splines are optimally positioned a distance away from the central opening such that the centrifugal forces acting on the rotor are substantially balanced when the rotor is in operation. This distance, or optimum mean radius, of the splines from the central axis is the location where the plies of the rotor that are cut to form the splines least resist shear deflection with the adjacent solid plies under centrifugal loading. Each spline is generally defined by a pair of sidewalls and a base portion. Each spline also preferably defines an acute angle between each sidewall and an imaginary plane extending perpendicularly from the generally planar surface of the base portion. The acute angle allows the rotor to radially adjust and center itself relative to the coupler during operation, particularly when thermal expansion generated during operation causes various cross-sections of the rotor to expand at different rates. The rotor also does not substantially slip in the circumferential direction relative to the coupler, thus further improving the torque carrying capability of the rotor.
As stated above, the splines are recessed into the body portion of the rotor, such as by machining. In particular, the splines are recessed to a controlled depth such that the structural integrity of the woven plies extending to form the blades is maintained. In this regard, the splines are recessed into the body portion such that the base portion of each spline is, at most, adjacent the woven plies that extend radially from the part of the body portion proximate the central opening to the blades.
In one embodiment, the rotor also includes fibers that are stitched into the body portion, i.e., into and through the axial planes defined by the woven fiber plies. Preferably, the stitched fibers are perpendicular to the woven fiber plies. The added fibers add strength to the inter-laminar shear strength of the rotor, thus improving its torque carrying capability. The added fibers are stitched into the body portion from its surface to a specified depth prior to forming the splines. For rotors having splines on both sides, the added stitched fibers can be included on both sides as well.
Thus, the rotor of the present invention overcomes several shortcomings of conventional rotors. For example, the composite rotor of the present invention provides a plurality of recessed splines that enable the rotor to transmit torque orders of magnitude greater than conventional FRCMC or metallic rotors in high temperature environments. In addition, the splines are formed to radially adjust and center the rotor relative to the coupler due to thermal expansion and centrifugal growth caused during operation. The splines also are formed to avoid the stress concentration points created by sharp corners from conventional machining processes. As such, the rotor of the present invention provides a significant advancement in the art and allows a broader range of applications for FRCMC components.